Library · Air, climate & CO₂
Frost Tolerance Reference.
Frost Tolerance Reference
When the forecast says 28°F tonight, what dies and what survives? Plant-by-plant frost survival temperatures across 50+ crops.
Frost terminology
| Term | Temperature range | Effect |
|---|---|---|
| Light frost | 30-32°F | Sensitive crops damaged; semi-hardy fine |
| Frost | 28-30°F | Tender crops killed; semi-hardy damaged |
| Hard freeze | 24-28°F | Most crops killed; hardy crops damaged |
| Killing freeze | Below 24°F | Even hardy crops at risk |
| Black freeze | Below ~22°F + dry/windy | Even very hardy crops affected; frost damages tissues without ice formation |
Microclimate & forecast accuracy
Forecast lows are typically taken at 5 ft above ground at official weather stations. Your garden may be 3-5°F colder than forecast because:
- Cold air pools in low spots, creating frost pockets
- Open sky / clear nights radiate heat to space; cloud cover acts as a blanket. Clear + calm = coldest
- Garden plants on bare soil are colder than plants over wet soil (water holds heat)
- Plants near south-facing walls or under tree canopy are warmer due to thermal mass + shelter
- Black plastic mulch absorbs heat day, releases at night — ~3-5°F warmer than bare soil
Practical rule: add a 5°F safety margin to forecast lows when deciding whether to cover or harvest.
Protection strategies
| Strategy | Protection | Notes |
|---|---|---|
| Light row cover (0.55 oz) | ~2-4°F | Cheap, easy; lay before sunset |
| Heavy row cover (1.5 oz) | ~6-8°F | Best balance; reusable |
| Double layer + low tunnel | ~10-15°F | For overwintering kale, spinach, etc. |
| Sprinkler irrigation overnight | ~5-8°F (down to 28°F) | Water freezes, releases heat. Run continuously through frost. Standard commercial orchard practice. |
| Wind machine / fan | ~3-5°F (only for radiation frost; no help with advective freeze) | Mixes warm air aloft with cold air below; only works in calm-air radiation frost |
| Smudge pots / orchard heaters | ~5-10°F over small area | Energy-intensive; restricted in many areas due to air quality |
| Cloches / Wall-O-Water | ~10-15°F | Individual plant protection; great for tomato in early spring |
| Mulch (heavy) | ~5°F (root zone only) | Protects perennial roots, not aerial parts |
| Greenhouse / high tunnel | ~5-15°F (unheated); unlimited (heated) | Major capital; permanent protection |
Free under CC BY 4.0. Cite as "OAT Frost Tolerance Reference (openagriculturetechnology.com)".